Treatment of wood and wood-based materials

ABSTRACT

A method of treating moisture containing wood including the step of exposing the timber or wood to a vaporous azeotrope of an organ-boron component and an alcohol at a temperature below the alcohol&#39;s boiling point. The organ-boron compound hydrolyses with the moisture to form boric acid in the timber or board.

This invention is concerned with methods for the preservative treatmentof timber and wood-based products e.g. wood-based boards, to offerprotection against rot, insect attack or to impart flame or fireresistance. The invention also embraces apparatus suitable for carryingout the method and materials treated by the process and/or in suchapparatus.

Many organo-boron compounds are gases or low boiling point liquids. Whenarranged to contact with timber or wood-based products, certain of thesecompounds hydrolyse with the wood moisture to release the boron as boricacid in the timber. For example, trimethyl borate (TMB) is believed toreact with moisture in wood to form boric acid by the reaction:

    B(OCH.sub.3).sub.3 +3H.sub.2 O→H.sub.3 BO.sub.3 +3CH.sub.3 OH

Thus, according to this reaction, the organo-boron compound trimethylborate is capable of hydrolysing to boric acid reaction product andother reaction product, which is methanol in this case. Of the tworeaction products, methanol has the lower boiling point about 64.7° C.at atmospheric pressure.

Trimethyl borate boils at about 68.5° to 69° C. at atmospheric pressure.In previously known treatments, application of the vapour at hightemperature required both the treatment vessel and the timber to beheated to prevent condensation of the vapour. Wood moisture content alsoaffected the quantity of trimethyl borate converted to boric acid.

Proposed treatments at working moisture contents of wood have been foundto be largely ineffective for bulk timber because of incompletepenetration of the TMB beyond a surface layer of the timber. Reductionof wood moisture content was found to increase penetration but fullpenetration was only found at reduced moisture contents which were belowpractical, working moisture contents. Timber dried to such levels cansuffer problems such as warping or splitting, which Would render suchtimber of little commercial value.

An object of the present invention is to obviate or mitigate theaforesaid disadvantages, and to provide a treatment and apparatus alsosuitable for wood-based board materials.

According to one aspect of the present invention, there is provided amethod of treating timber or wood based board comprising exposing saidtimber or board to vapour derived from a mixture comprising anorgano-boron compound and a second compound, said compounds beingcapable of forming a positive azeotrope if mixed in suitable molarproportions, said organo-boron compound hydrolysing to boric acidreaction product in said timber or board and other reaction product, thevapour exposure being effected at a temperature which, under thetreatment conditions selected, is greater than or equal to the boilingpoint of the mixture used, but below the boiling point of said otherreaction product.

The treatment temperature, under the selected treatment conditions ofe.g. reduced initial pressure, wood or board type, moisture content,desired level of boric acid penetration, is therefore most preferablycapable of generating vapour from the mixture but of suppressingvaporisation of the other reaction product, being suppression ofvaporisation of methanol reaction product in the case that TMB is usedas the organo-boron compound. It has been found that commerciallyavailable positively azeotropic liquid mixture of TMB/methanol performswell in the present invention, comprising approximately equi-molarproportions of these two compounds, and having a boiling point lowerthan both individual compounds.

The molar proportion of the second compound may vary and it is preferredto use mixtures whereby the molar amount of second compound is from 10%to 90%, more preferably at or near the azeotropic molar percentage.

According to an embodiment of the present invention there is provided amethod of treating timber or wood based board comprising exposing thetimber or wood based board to the vapour of a positive azeotrope of aliquid organo-boron compound, which compound is hydrolysable to boricacid, with a second liquid, said treatment being effected at atemperature above the boiling point of (i) the azeotropic mixture, butbelow the boiling point of (ii) the reaction product with the lowerboiling point under the prevailing treatment conditions.

It is further preferred that the treatment is effected at a temperaturewhich is also below the boiling point of (iii) the individual azeotropeconstituents under the prevailing treatment conditions.

Apparatus, suitable for carrying out the present method, comprises atreatment chamber capable of receiving wood or wood based board and ofbeing partially or substantially evacuated, means associated with thechamber for ascertaining the temperature and/or pressure therein, areservoir for containing the mixture in gaseous or liquid communicationwith the treatment chamber, means permitting continuous presence ofmixture vapour in said treatment chamber, and means for altering thetreatment chamber pressure and/or temperature.

It is preferred that the treatment apparatus i.e. treatment chamber,mixture reservoir and connecting means, e.g. pipes are maintained at thesame temperature to maintain equilibrium between the liquid and gasphases during treatment.

The treatment can be carried out at any suitable temperature and/orpressure providing the above stated temperature and boiling pointrelationship is maintained.

For example only, treatments may be carried out at a temperature in therange of -20° C. to 75° C., preferably in the range of 10° C. to lessthan 64.7° C., and at an initial reduced pressure in the range of 750mbar to less than 1 mbar, preferably in the range of 500 mbar to lessthan 1 mbar. Treatment may be carried out at an initial increasedpressure.

The organo-boron compound is preferably an alkyl borate such astrimethyl borate [B(OCH₃)₃ ].

The most preferred organo-boron compound is trimethyl borate (TMB) andthe other compound is preferably methanol. However, other liquidsforming a binary or, indeed, ternary azeotrope with the organo-boroncompound may be used. The second compound used is conveniently a liquid.

Treatment of timber or board can be carried out to achieve partialpenetration of boric acid into said timber or board, wherein suchpartial penetration may be about 5% to 25% of the thickness and/or depthof the timber or board.

Preferably the treatment is continued for a time sufficient to depositin the timber or board a concentration of boric acid of not more than 3%by weight, and preferably from 0.1 to 1% by weight, for preservativetreatment or from 3 to 20% by weight for flameproofing or fireproofing.

The moisture content of the board and/or timber prior to vapourtreatment may be in the range 0-28%, preferably 2-20% for boards, and6-20% for timber. Wood based boards can be treated at their workingmoisture contents, i.e. in the range 4% to 12%.

Preferred treatment involves introduction of mixture vapour, e.g.azeotrope vapour into a treatment chamber which is pre-evacuated, toachieve an initial vacuum before vapour treatment.

The initial vacuum, if applied, may be in the range from 500 to lessthan 1 mbar, more preferably 100 to less than 1 mbar. The vacuum is mostpreferably applied prior to introduction of the boron preservative i.e.mixture vapour. We believe that since the vapour pressure of the presentmixture can exceed the vapour pressure of the reaction products,vaporisation of the other reaction product (e.g. principally methanol)can be effectively suppressed.

In the above reaction between organo-boron compound and moisture a large(3 times) molar excess of other reaction product is produced.

Vaporisation of this other reaction product, (e.g. methanol) wouldincrease the reaction pressure and consequently inhibit furthervaporisation of the organo-boron compound. This, we believe, markedlyreduces the efficiency of treatment of wood or wood based boards eitherat or below normal working moisture contents by severely limiting theavailable organo boron gas concentration. In contradistinction, by meansof the present method we believe that the methanol tends topreferentially condense as liquid in the timber or wood based boards,i.e. its vapour suppression enables considerably improved boronpreservative vaporisation (derived from the present mixture) therebysurprisingly improving the efficiency of boric acid deposition.

By using treatments according to the invention, the vapour concentrationderived from the mixture can be maintained at a maximum practical levelthroughout the treatment time selected. This enables continuousreplenishment of mixture vapour during the treatment; a most preferredaspect of the present treatment as exemplified below.

This continuous replenishment of vapour comprising the organo-boroncompound, can be achieved by maintaining gaseous communication betweenthe reservoir of mixture and the treatment vessel or by providing liquidcommunication therebetween such that vaporisation takes place in thetreatment chamber for the treatment time selected. As the reactionproceeds between TMB and the water in the wood or wood based board, gasconcentration decreases, the vacuum increases drawing more mixturevapour into the chamber, eventually reaching an equilibrium butproviding an almost unlimited supply of organo-boron preservative in thevapour.

The treatment time may be dependent on the various treatment conditionsand may be selected on the basis of desired boric acid retention.

In certain embodiments of the present invention, useful for treatingtimber, the solid wood can be treated at its working moisture content,as described previously.

Such embodiments for treating solid wood can be devised which avoid theneed to (a) pre-condition the untreated wood to a moisture content belowworking moisture content and/or (b) the need to post-condition thetreated wood to a practical working moisture content for its intendedfinal use. Depending upon treatment conditions it may alternatively bedesirable to pre-condition by heating to reduce the pre-treatmentmoisture content and/or post-condition to increase the moisture contente.g. by steam conditioning. Such conditioning techniques are known inthe timber processing art and the present invention embraces treatmentof wood or wood based products which either have or have not undergonemoisture content alteration.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention in its various aspects may beillustrated and readily carried into effect, non-limiting embodimentsthereof will now be described by way of example only, with reference tothe accompanying drawings in which:

FIG. 1 shows a liquid/vapour phase diagram for mixtures of TMB/methanolat atmospheric pressure, and

FIG. 2 shows one form of apparatus, suitable for carrying out treatment.

FIG. 1 of the drawings herewith shows a phase diagram for trimethylborate/methanol mixtures at atmospheric pressure. From FIG. 1 it will beseen that the minimum boiling point (54.3° C.) of an azeotropic mixtureof the two compounds occurs at equi-molar proportions. The boilingpointof methanol is about 64.7° C. and that of TMB is about 68.5° C. Usingthis particularly preferred azeotrope, therefore, requires a treatmenttemperature below 64.7° C. but at or above 54.3° C. at atmosphericpressure. Equivalent temperatures and pressures could be used as definedby the vapour pressure/temperature relationship for the mixture.

One suitable form of small scale treatment plant shown in FIG. 2consists of an internal treatment chamber 1 contained within anenvironmental chamber 2, the temperature of which could be accuratelycontrolled over a range from -70° C. through to +200° C. (+/-0.1° C.accuracy).

The internal treatment chamber can be cylindrical and constructed ofsteel tubing and stainless steel plates used for the end plate, flangeand lid of the cylinder. End plate and flange can be welded to ensure avacuum tight fit. Two pins can be placed in the flange to locate the lidwhen sealing the chamber. A handle may be attached to the outside of thelid tofacilitate handling while on the inside, a silicone rubber "O"ring can be used in a machined groove to ensure a vacuum tight sealbetween lid and flange. The whole cylinder was fastened in a cradle forstability.

Four ports (3-6) were drilled and tapped in the cylinder wall viastainlesssteel bosses to accommodate:

a thermocouple at port 3 linked to a digital thermometer (accuracy +/-1°C. not shown),

a pipe 10, at port 4, linking a reservoir 7 of TMB/methanol mixture invapour communication with the main treatment chamber 1,

a pipe 11, at port 5, connected to a vacuum pump, and

a pressure transducer (not shown), at port 6, linked to

a digital gauge (not shown, accuracy +/-1 mbar) to determine the vacuumlevel within the treatment chamber.

The pipe 11 connecting the main treatment chamber 1 to the vacuum pump(notshown) and a tap 9 controlling gaseous vapour flow from theTMB/methanol mixture reservoir 7 and the treatment chamber I passthrough the wall of the environmental chamber for easy adjustments. Avalve 8 operated by tap 9 is located in the vapour communication pipe 10between container 7 and chamber, to permit evacuation prior to vapourexposure.

PROTOCOLS

The materials used were;

1. Oriented Strand Board (OSB), 18 mm thick, which had an equilibriummoisture content in the laboratory of approximately 6%.

2. Flooring grade chipboard (18 mm) with a moisture content of 10%.

3. Solid wood of the slow grown Pinus sylvestris which was conditionedto arange of moisture contents of from 6 to 12%.

The board samples were cut to dimensions 100 mm×100 mm×board thicknessand edge sealed with an ABS polymer before treatment The solid wood wascut to 50×50 mm cross section×160 mm length and the ends sealed withepoxy resin.

After conditioning of the sample specimens to known moisture content, ifrequired, they were placed in a treatment chamber at a selectedtemperature which was then sealed and the samples allowed to equilibrateto the ambient temperature therein.

The combination of treatment temperature and pressure was selected suchthat at least some organo-boron compound would be in the vapour phase aspart of the mixture vapour. Thereafter, a valve connecting the treatmentchamber to a reservoir of treatment material (either TMB alone, forcomparison purposes, or the preferred TMB/methanol azeotrope), wasopened allowing vapour to enter the chamber. The exposure to the vapourwas maintained for a selected period of time.

At the end of the treatment time the increase in pressure was recorded,andthe chamber vented to atmosphere and purged with nitrogen to expelresidualvapour.

The treated specimens were weighed to determine the weight increasecaused by deposition of boric acid. Distribution of boric acid withinthe specimens was assessed visually after spraying a centrally cutcross-section with a staining reagent consisting of 0.25 g of curcuminand10 g of salicylic acid dissolved in 10 ml of ethanol. This stainreveals boric acid above 0.2% w/w as a red colouration (BritishStandard: 5666 part 2, 1980).

The loading of boric acid was also determined quantitatively by themethod described by Williams [Analyst 93: 111-115 (1968) and Analyst,95: 498-504(1970)].

EXAMPLE 1

Table I below summarises the influence of temperature and moisturecontent on retention and penetration in solid wood using the azeotropeof TMB and methanol according to the invention, and, for comparison,pure TMB. The treatment time was four hours.

                  TABLE I                                                         ______________________________________                                                       99% TMB   Pene- Azeotrope                                      Temp  Moisture Retention tration                                                                             Retention                                                                             Penetration                            °C.                                                                          % (dry)  % (dry)   (mm)  % (dry) (mm)                                   ______________________________________                                        20    12       2.9       3.7   4.4     5.1                                    50    12       5.8       6.0   10.9    8.0                                    50    10                       11.3    11.0                                   55    10       7.9       9.2   11.4    12.2                                   65     8                       11.6    14.6                                   65     6       7.8       14.2  10.4    18.3                                   ______________________________________                                    

Retention values quoted are the mean of five replicates and are given asincrease over the dry weight of the specimens.

The data in table 1 illustrate increased retention and penetrationachievedwith a mixture according to the invention, compared with TMBalone. It is also noted that whilst a partial impregnation of the timbersamples is achieved under all the treatment conditions selected, the useof a vapour mixture of organo-boron compound and second compoundprovides a markedly superior degree of penetration. It is particularlysurprising and therefore advantageous that an improved level ofpenetration i.e. better partial impregnation with preservative at lowertemperature e.g. 20° C., and at higher moisture content e.g. 12% isobtainable.

The penetration levels achievable with the present mixture, andparticularly with the preferred azeotropic mixture, at these temperatureand moisture levels may be quite satisfactory for certain end uses ofthe wood, or board.

EXAMPLE 2

Table II below summarises the effect of treatment time on the boric acidretention for Oriented Strand Board (OSB), of moisture content 6%, usingthe azeotrope in accordance with this invention and, for comparison,pure TMB.

                  TABLE II                                                        ______________________________________                                                       At 50° C.    At 20° C.                           Time  Azeo-    RETENTION           RETENTION                                  (min) trope    99% TMB     Azeotrope                                                                             99% TMB                                    ______________________________________                                         1    0.3                  0.2                                                 5    1.5      1.0         0.8     0.5                                        10    2.0      1.5         1.3     0.8                                        20    3.1                  1.8                                                45    4.8                  2.7                                                ______________________________________                                    

Full penetration of all samples was observed. Quantitative determinationofthe 10 and 20 minute samples for the azeotrope gave 2.2% and 1.5% at50° and 20° respectively and 3.0% and 2.0% at 50° and20° C.

EXAMPLE 3

Specimens of 18 mm chipboard (BS:5669 Type ii/iii) of moisture 10% weretreated to retentions of boric acid consistent with its use as a flameretardant by exposure to the TMB/methanol azeotrope at 50° C. Theresults of varying the treatment time are reported in Table III below.

                  TABLE III                                                       ______________________________________                                               Time  Retention                                                               (mins)                                                                              (%)                                                              ______________________________________                                               30    4.1                                                                     60    6.2                                                                     120   7.6                                                              ______________________________________                                    

Full penetration was observed in all specimens.

Treatment of other board materials, e.g. MDF, OSB, has achieved boricacid retentions up to 14% and 18%, respectively, at appropriate boardmoisture contents and treatment conditions.

From the results quoted in Example 2 above, it will be seen that for OSBthe azeotrope treatment confers no particular advantage over pure TMB asfar as penetration is concerned since full penetration was observed withboth treatments: the advantage lies in the increased loading of boricacidachieved by use of the process of the invention.

As far as solid timber is concerned (Table I), improvements in bothloadingand penetration by use of the process of the invention areachieved. It is expected that full penetration across a 50 mm×50 mmcross section pine will be achievable, given optimised treatmentconditions.

Boric acid has many properties which make it ideal for use as apreservative for wood based board materials:

1. Proven effectiveness against decay fungi and insects.

2. Low mammalian toxicity.

3. Minimal vapour pressure.

4. Colourless.

5. No deleterious effects on wood.

The commonly cited disadvantage of the leachability of borate is notconsidered to be problematical in the present application since mostwood based boards are not intended for use in situations of highleaching hazard. This treatment can be used with manufactured boards andthereby may avoid another potential disadvantage in board treatments,namely that of interference of the preservative with the bonding of theboard during manufacture. The application of preservatives to boardmaterials after manufacture allows fabrication to proceed under optimalproduction conditions and has a secondary advantage in that a varyingproportion of board output can be treated in response to demand forpreserved boards.

The present method can produce boards ready for use immediately aftertreatment.

For the majority of boards, e.g. OSB--MDF, Chipboard, Waferboard etc.moisture level conditioning is not necessary pre- and post- vapourtreatment. After manufacture these boards generally have an appropriatemoisture content at the production site where vapour treatment might becarried out particularly economically by virtue of reductions in energyand transportation costs. Of course, the invention can still be used fortreatment of boards which have achieved an equilibrium moisture contentinstorage or are conditioned to achieve a working moisture content aspart ofthe board production process.

We claim:
 1. A method of preserving timber or wood based boardcomprising exposing said timber or board having a moisture content up to28% by weight to a vapor mixture comprised of a positive azeotrope of anorgano-boron compound and an alcohol at a temperature greater than orequal to the boiling point of the mixture, but below the boiling pointof the alcohol, wherein said organo-boron compound hydrolyses to form aboric acid reaction product in said timber or board.
 2. A method asclaimed in claim 1 in which said organo-boron compound comprisestrimethyl borate and said alcohol comprises methanol.
 3. A method asclaimed in claim 1 further comprising the formation of methanol as areaction product.
 4. A method as claimed in claim 1 wherein the methodis carried out in a partial vacuum.
 5. A method as claimed in claim 1wherein the vapor exposure is effected at a temperature which is alsobelow the boiling point of said organ-boron compound.
 6. A method as inclaim 1 wherein the treatment is effected to achieve partial penetrationof boric acid into said timber or board.
 7. A method as claimed in claim1, wherein said temperature is in the range of -20° C. to 75° C.
 8. Amethod as claimed in claim 7, wherein the treatment temperature T, atatmospheric pressure, is in the range 54.3° C.≦T<64.7° C.
 9. A method asin claim 1 which uses an apparatus, said apparatus comprising atreatment chamber capable of receiving wood or wood based board and ofbeing at least partially evacuated, means for ascertaining thetemperature or pressure therein, a reservoir for containing a mixture ofan organo-boron compound and a second compound, means for gaseous orliquid communication between said reservoir and said chamber, and ameans for adjusting pressure or temperature, characterized in that saidtimber or board is exposed in the apparatus
 1. 10. The method of claim9, wherein the treatment chamber, mixture reservoir and communicationmeans are maintained at the same temperature.
 11. The method of claim10, wherein the apparatus is housed in a chamber.
 12. Timber orwood-based board, treated by a method as claimed in claim
 1. 13. Timberor wood-based board, treated by a method as claimed in claim
 14. Amethod as claimed in claim 1 wherein said mixture comprises anazeotropic mixture of trimethyl borate and methanol.
 15. A method asclaimed in claim 1 wherein the method is carried out at a moisturecontent of said timber in the range of 6 to 20 percent by weight.
 16. Amethod as claimed in claim 1 wherein the method is carried out at amoisture content of said wood based board in the range of 2 to 20percent by weight.
 17. A method as claimed in claim 1 wherein thetreatment temperature is in the range of 10° C. to <64.7° C.
 18. Amethod of preserving timber or wood based board comprising partiallydrying said timber or board having a moisture content up to 28% byweight, exposing said partially dried timber or board to a vapor mixturecomprised of a positive azeotrope of an organo-boron compound and analcohol at a temperature greater than or equal to the boiling point ofthe mixture, but below the boiling point of the alcohol, wherein saidorgano-boron compound hydrolyses to form a boric acid reaction productin said timber or board.
 19. A method of preserving timber or wood basedboard comprising exposing said timber or board to a vaporized azeotropecomprised of trimethyl borate and methanol at a temperature greater thanor equal to the boiling point of the azeotrope but below the boilingpoint of the methanol, wherein said trimethyl borate hydrolyses to formboric acid in said timber or board and methanol.
 20. The method of claim19 conducted at atmospheric pressure.
 21. The method of claim 20,wherein said temperature is greater than or equal to about 54.3° C. andless than about 64.7° C.